1. Field of the Invention
The present invention relates to a backlighting device used as a surface-area light source for various kinds of display devices, and also relates to a display device, and particularly a color display device, that is used, for example, in transport means such as an automobile, vessel train car, and that allows the driver or operator of the transport means to view images or characters displayed on the display in superimposed fashion on an outside view ahead.
2. Description of the Related Art
In recent years, an edge lighting method has become a predominant method for flat display backlighting. This method uses a substrate made of an acrylic resin or the like, and a light source (such as a fluorescent lamp or cold-cathode lamp) disposed in close proximity to an edge of the substrate, and light from the light source is introduced into the transparent resin substrate and is made to emerge from the display surface. If a picture or other visual means is used instead of a display, this edge lighting method can be used for a decorative panel or a signboard. In such applications, in order to utilize efficiently light it is usual to place a reflecting plate on the reverse side of the substrate and a diffusing plate on the front side.
The refractive index of the acrylic resin (polymethyl methacrylate) is n=1.49. Therefore the critical angle of reflection at the interface with air is about 42.degree., and most of the light from the light source propagates through the substrate without emerging from the substrate. A variety of methods have been proposed to avoid this limitation and to extract the light in the desired direction (Japanese Patent Unexamined Publication Nos. 61-55684 and 1-245220, Japanese Patent Examined Publication No. 58-17957, Japanese Patent Unexamined Publication No. 59-194302, etc.).
In a method that involves applying a white paint or the like on the front side to diffuse light, light absorption by the paint cannot be ignored. Another method involves roughening the front surface from which the light emerges, but with this method also, sufficient light utilization cannot be obtained. In most of the currently practiced methods, direct emergent light from the front surface is not actively utilized, but a reflective film is placed on the reverse side of a light guiding plate and reflected light from the reflective film is made to emerge from the front surface. A white-colored film (including a white color painted plate, white color printed film, etc.) or a metallized film is used as the reflective film. The diffusing plate on the front side is used presumably for the purpose of evening out the reflected light, and furthermore, to enhance the directionality of the emergent light and increase its brightness, a method that uses a prism film or the like is employed.
However, with the shift from the traditional monochrome display to the more advanced full-color display, it is becoming increasingly important to increase the screen brightness, and a further improvement in the efficiency of backlighting is needed. To achieve this, various improvements have been proposed, such as reducing the cold-cathode lamp diameter, improving the reflective film, and improving the geometrical shape of the light guiding plate. While these improvements have been successful in increasing the brightness, they have had other problems such as increased cost and increased complexity of the system. Furthermore, utilizing reflected light is reaching its technological limit. The only way left to further improve the efficiency is to utilize the directly emerging light from the front surface of the light guiding plate. Utilizing the direct light from the front surface involves the problem of using the surface-area light source as a transparent body.
In applications where a picture or other visual means is used instead of a display to create a display apparatus (static) such as a light-guiding type decorative panel or signboard, as earlier described, in the case of displaying a color picture the color picture can be used as is, and in other cases, usually a light-scattering image or sign pattern is drawn and cut and is overlaid on a background color plate, while using a color filter to accomplish a color display.
On the other hand, if a dynamic display is to be achieved with this display, one possible way will be to use a light-scattering image using, for example, a macromolecular dispersion-type liquid crystal (polymer dispersed liquid crystal). In connection with this method, constructions similar to the above display apparatus are disclosed, for example, in Japanese Patent Unexamined Publication Nos. 3-73926 and 3-23423. The former proposes arranging different-colored light sources on two edge sides of a liquid crystal panel and switching the color between the two light sources thereby changing the color of a single image in time sequence. The latter states that a color display can be realized using color filters though no specific examples are shown.
In the (static) display apparatus of the above prior art, since color filters are used for the image portion, the number of colors that can be produced is fixed. Furthermore, with the exception of special examples, since a reflecting plate or a colored screen or the like is used for the background, they generally lack the concept that the whole construction should be made transparent. On the other hand, in the dynamic display apparatus using a macromolecular dispersion-type liquid crystal, the image can be displayed in mono-color but cannot be displayed in multi-colors at any arbitrary time and in any arbitrary region. Japanese Patent Unexamined Publication Nos. 3-73926 and 3-23423, cited above, both use a reflecting plate or a colored absorbing plate in their embodiments and lack the concept that the whole construction should be made transparent. The freedom of color selection can be increased and multi-color capability achieved by making the construction transparent and by stacking one on top of another a plurality of display devices each used to introduce a different color. In this case, however, the problem of how to separate the respective colors has to be addressed. There is also the problem of how to increase the variety of colors. A further problem is how these requirements can be accomplished in a compact and efficient construction.
There are two main types of display device: the self-luminous type such as the CRT, PDP, LED, and EL, and the external light type, such as the liquid crystal and electrochromic display, that produces the display by receiving light from a separate light source. In some applications, such display devices are positioned for use near a window glass or a show window. In other and more recent applications, such display devices are used as instrument and other display devices in automobiles, vessels, train cars, etc., a typical example being a direct-view head-up display (HUD). Since the display area of any of these display devices displays information thereon by emitting some form of display light, if the display device is positioned within such a distance that the display light reaches a window glass or a show window, the displayed image is reflected more or less onto the window glass or show window and comes into the viewer's visual field. The phenomenon of such reflection also occurs on a window glass or a show window near which a signboard or the like is put up.
In such cases, when light from the background of the window glass or illumination inside the show window is sufficiently strong, the reflection hardly disrupts the viewer's view, but when the light from the background of the window glass or the illumination inside the show window is weak, or when the display light from the display device is very strong, the reflection on the window glass or the show window becomes pronounced and annoys the viewer, and in the case of vehicular applications, adversely affects the driving operation of the driver.
Traditional methods to prevent such reflection have been to position the display device so that the display area does not face the window glass or the show window, to block the display light using a shade so that the display light does not fall on the window glass, to reduce the ambient light as well as the brightness of the display to reduce the intensity of the display light, and so on. For example, in a train or a bus running in the nighttime, in order that the panel faces such as instrument panels in the cab may not be reflected onto the windshield by the illumination from the passenger compartment and be brought into the visual field of the operator or driver, a shade is provided around the cab to block the illumination from the passenger compartment from entering the cab, or the lighting in and around the cab is dimmed so that the panel faces such as instrument panels in the cab are not reflected by the lighting onto the windshield and brought into the visual field of the operator or driver.
However, the methods employed to prevent the reflection on the window glass or the show window involve the following problems.
In the method that positions the display device so that the display area does not face the window glass or the show window, the position where the display device can be placed is very limited, and particularly, if the display device is positioned against the-window glass, the display become very difficult to view when the outside light is bright, though it is easy to view when the outside light is weak.
In the method that provides a shade on the display device, a very wide shade may become necessary depending on the positional relationship between the display device and the window glass or the show window, and often the use of such a shade is not permitted by space limitations or design considerations.
In the method to reduce the ambient light and reduce the intensity of the display light, it becomes necessary to protect the area around the display device from light as the display would become difficult to view if the light through the window glass or the show window is incident on the display device. The display becomes very difficult to view particularly when the outside light from the window glass or the show window is bright.
Instrument panels in passenger cars are a typical example where reflections cause a serious problem. The instrument panel of a passenger car is mounted in a recess in the dashboard, and the surrounding areas are painted black or dark color, to prevent the panel illumination from being reflected onto the windshield during night driving and to prevent outside light from falling directly on the instrument panel during day driving. In such applications where the intensity of the ambient light varies greatly, the method of installing the display device is demanding and very limited in freedom. Furthermore, the display can be viewed from a specified position but cannot be viewed when the viewer's head is moved, if slightly, from the specified position.
Liquid-crystal displays constructed with twisted nematic (TN) liquid crystals sandwiched between two substrates having transparent electrodes have been widely used. The structure and the operating principle of this type of liquid crystal display are shown in FIG. 39. When the liquid crystal molecules on the surface of a substrate 3900 are forced to align in one direction with their long axes arranged at 90.degree. with respect to the liquid crystal molecules on the surface of a substrate 3901 on the opposite side, as shown in FIG. 39(a) the liquid crystal molecules 3902 are arranged gradually changing their orientation and twisting through 90.degree. between the substrates. A polarizing plate 3903 with its direction of polarization indicated by arrow "a" and a polarizing plate 3904 with its direction of polarization indicated by arrow "b" are attached to the outer surfaces of the substrates 3900 and 3901, respectively. When light is incident on this structure, only light vibrating in the same direction as the direction of polarization of the polarizing plate 3903 is passed through it, and follows the twisted structure of the liquid crystal molecules, the polarization of the light thus being rotated through 90.degree. until reaching the polarizing plate 3904 on the opposite side. Since the direction of polarization of the light now coincides with that of the polarizing plate 3904, the light is passed through it.
On the other hand, when a voltage is applied between the electrodes on the substrates, as shown in FIG. 39(b), the liquid crystal molecules align themselves with their long axis direction parallel to the direction of the resulting electric field, so that the light entering the liquid crystal layer is passed through it with the direction of polarization of the light remaining unchanged and reaches the polarizing plate 3904 on the opposite side where the light is blocked, creating a dark spot. An image or character is thus displayed by applying a voltage at a portion where the image or character is to be displayed.
FIG. 40 shows another type of liquid crystal display which uses a macromolecular dispersion-type liquid crystal. In the macromolecular dispersion-type liquid crystal, the orientation of the liquid crystal molecules 4006 dispersed in the form of fine droplets in a macromolecular matrix 4005 is changed by applying an electric field, and the resulting change of refractive index is utilized to produce a display.
In the OFF state in which no voltage is applied, as shown in FIG. 40(a), the optical axes of the liquid crystal molecules 4006 are random, and the difference in refractive index between macromolecule and liquid crystal causes the entering light to scatter in many directions.
On the other hand, when the voltage is applied, that is, in the ON state shown in FIG. 40(b), the liquid crystal molecules 4006 are aligned in the direction of the applied electric field, because refractive indices of macromolecular and liquid crystal are substantially same so that the entering light is passed through without being scattered. That is, in the OFF state, the display is like frosted glass because of scattering light, and in the ON state, the display is like transparent glass. In this way, by placing image or character portions in the OFF state and other portions in the ON state, an image or character is displayed on a transparent screen.
However, the conventional twisted nematic liquid crystal display requires the use of two polarizing plates, as earlier described, and therefore, the light transmittance is 40 to 50% at most. This low transmittance is not suitable for forming an image on a transparent display screen, and it is difficult to display an image or character in superimposed fashion on the background view.
On the other hand, in the case of the macromolecular dispersion-type liquid crystal display which does not require the use of polarizing plates, the transmittance in the ON state is nearly 80 to 90%, which provides good transparency. However, when displaying patterns such as shown in FIG. 41, portions corresponding to the patterns to be displayed are placed in the OFF state to form scattered light images, while the other portions are placed in the ON state and thus made transparent, but since the wiring patterns for the display patterns placed in the OFF state also cause scattering of light, making the wiring patterns visible on the screen, visibility and transparency are impaired when the displayed images or characters are superimposed on the background view; this presents a problem particularly when displaying a plurality of patterns requiring an increased number of wiring patterns.
Furthermore, when an AC voltage is applied between the electrodes, a slight amount of current flows as this system is a kind of capacitor. When displaying patterns such as shown in FIG. 42, larger current flows through the wiring for the larger display pattern, and hence ia&gt;ib. Voltage drops Va and Vb through the respective wiring patterns are expressed by Va=r*ia and Vb=r*ib. Assuming the wiring patterns have the same resistance r, since ia&gt;ib, it follows that Va&gt;Vb, which indicates that the voltage drop is larger for the larger display pattern.
FIG. 43 is a voltage-transmittance characteristic diagram for a typical macromolecular dispersion-type liquid crystal, plotting the transmittance as a function of the applied voltage. As described above, if the same voltage is applied to the respective display patterns from the drive circuit, the voltages actually applied to the respective liquid crystal portions are different. This gives rise to the problem that variations are caused in the transparency of the screen and the transparent appearance is impaired.
Furthermore, if the area sizes of the display patterns are the same, as shown in FIG. 44, the resistance r is larger for the longer wiring pattern, which means a larger voltage drop. This also presents the problem that variations are caused in the transparency of the screen and the transparent appearance is impaired.
In view of the problems that have made it difficult to use the above-described prior art liquid crystal displays as transparent displays, it is an object of the present invention to provide a transparent liquid crystal display that can display only images and characters that are to be displayed, on a transparent screen in superimposed fashion on the background view, thereby providing good visibility.
For an automotive head-up display (HUD), a projection method is generally used wherein a CRT or a transmission-type liquid crystal display, for example, is used as the image source, and using optics consisting of a lens, mirror, and hologram, a display image or character produced on the image source is projected onto a combiner positioned in front of the driver's seat (or on the windshield) and displayed in superimposed fashion on a forward view. In this projection method, the image source is separately mounted, and a prescribed light path is necessary for projecting the image. The resulting problem is that the system becomes large in size and requires large space for installation, for example, in the dashboard.
Furthermore, in recent automotive instrumentation, a combination instrumentation system is used that displays warning indications, etc. as necessary in superimposed fashion on the ordinary speedometer and tachometer displays. Since this system uses the so-called virtual image display method, the system is complex, occupies large space for installation, and is expensive, which has been a major problem with this system.